1. Field of the Invention
The present invention relates to a seek control method, seek control device and medium storage device for performing seek control, then switching to following control, and more particularly to a seek control method, seek control device and medium storage device for preventing the step difference of current which occurs at switching from seek control to following control.
2. Description of the Related Art
A device for controlling the position of an object to a target position is widely used. For example, this device is used for seek control (and following control performed thereafter) for moving a head to a target track, which is one of the positioning controls of a medium storage device, such as a magnetic disk device and an optical disk device.
In the seek control, coarse control is performed, which is switched to fine control near a target position, so that both high-speed moving to the target position and high precision positioning are implemented. For this, different control characteristics are provided for the coarse control (seek control) and fine control (following control).
FIG. 21 is a block diagram depicting a position control system of a prior art. A controller 100 controls a plant (object) 110. The controller 100 has a position error computing section 112 for computing a position error e between an actual position of the plant 110 and a target position Lseek, a target trajectory generation section 102 for generating a target trajectory r(n) based on the target position (or moving distance) Lseek, two controllers 103 and 104, a settling judgment section 108, a position error switching section 109, and an output switching section 106.
According to this position control system, in seek control, the controller 103 for seeking calculates the output current based on the target trajectory r(n) from the target trajectory generation section 102 and the position error ‘e’ from the switching section 109, and drives the plant 110 via the switching section 106.
On the other hand, the settling judgment section 108 judges whether the position error ‘e’ satisfies a predetermined settling judgment condition (e.g. zero), and if judged that the settling judgment condition is satisfied, the settling judgment section 108 judges that the object reached near the target position, and switches the two switching sections 106 and 109 to the controller 104 for following.
By this, following control starts, and the controller 104 for following calculates the output current value based on the position error ‘e’ from the switching section 109, and drives the plant 110 via the switching section 106 (e.g. see Japanese Patent Application Laid-Open No. H03-233609 and Japanese Patent Application Laid-Open No. H05-143165).
In this way, according to this configuration, the characteristics of the control system are different between the seek control and the following control. For example, during seek control, a configuration of a 400 Hz band filter is used, and during following control, a configuration of an 800 Hz band filter is used.
During seek control, the head passes many tracks at high-speed in the disk device, so noise included in the observation position is high compared with the following control period. If the seek control is performed with a same control band as the following control, the output current is easily influenced by the noise and designed characteristics cannot be implemented, so the control band is dropped during the seek control so as to prevent the influence of the noise.
FIG. 22 is a diagram depicting the problem of the prior art. As FIG. 22 shows, if a position error PES(e) becomes a predetermined threshold Th (e.g. ±0.5 track from the center of a target position in a magnetic disk device) or less, the settling judgment section switches the seek control to the following control.
In this case, even if current for controlling the same position error PES to zero is calculated at switching, the control bands of the seek control system and the following control system are different, so the output current changes at switching, where the step difference of output current is generated. This step difference of the output current is an instantaneous step change, so the operation of the actuator changes greatly in an instant, and a low audible sound is generated.
Since such devices as disk storage devices have become popular lately, storage devices are used in the home electronic field. For example, storage devices are used for portable music players, portable telephones, notebook type personal computers and video recording devices.
When a storage device is used for this equipment, low noise design is required, that is it is demanded that the sound is as low as possible. Therefore the audible sound generated by the current step difference, previously not a problem, now must be prevented.
Further the generation of the current step difference causes the generation of residual vibration since the following control system performs control to cancel this step difference. In other words, the time until following stabilizes and positioning completes takes longer.